Hitherto, in a long-distance wireless transmission route, for example, reception level fluctuates significantly depending on place, time and polarization, generally, due to the occurrence of fading, and it has been attempted to prevent fluctuations of reception level by employing diversity technology. Conventional diversity antennas are shown in FIG. 30A and FIG. 30B.
FIG. 30A shows a space diversity antenna having four monopole antennas 101 disposed perpendicularly on a ground plate 100 at a specific interval. In each monopole antenna 101, received signal levels are compared, and the higher one is selected, and deep attenuation of reception signal caused depending on place of reception or the like can be lessened. To enhance the effect of space diversity, it is required to lower the correlation coefficient by extending the mutual distance of antennas.
FIG. 30B shows a directive diversity antenna having a first dipole antenna 102 and a second dipole antenna 103 disposed orthogonally so that the directivity of each antenna may cross orthogonally. Since fading occurs in every polarized wave, in one place, for example, a vertical polarized wave is not received at all while a horizontal polarized wave is received by a large reception power. In such a case, by using a directive diversity antenna, deep attenuation of reception power can be lessened.
However, when the space diversity antenna in FIG. 30A is applied in a mobile terminal, it is extremely difficult to keep a specific distance among antennas in the recent downsizing trend of mobile terminals. In a small portable terminal, if antennas are closely disposed to each other to realize a space diversity, since the directivity pattern on the horizontal plane of each monopole antenna 101 in FIG. 30A is nondirectional, arbitrary incoming waves are similarly received by the antennas and it is highly possible that the reception voltages of the antennas be identical, and the correlation coefficient of the monopole antennas may deteriorate significantly.
Or, when the directive diversity antennas in FIG. 30B are disposed parallel to each other on the ground, the bandwidth becomes narrow, and the antenna gain deteriorates extremely. It is hence difficult to mount the antennas on the ground, which is the basic requirement for realizing incorporation of an antenna in a small portable terminal, and directive diversity may not be realized in a small portable terminal. Besides, since the antenna is made of a metal element, it is hard to retain the shape and is likely to be broken.